Steering system, azimuthing propulsion system, and method for absorbing heat

ABSTRACT

According to an example aspect of the present invention, there is provided a steering system of an azimuthing propulsion system, the steering system comprising at least one hydraulic motor configured to operate an azimuthing system of a propulsion unit, the propulsion unit being arranged outside a vessel, a fluid cycle from the at least one hydraulic motor via a separate hydraulic overload protection unit and back to the motor, the overload protection unit comprises a pressure relief unit and a heat management unit, and wherein the pressure relief unit comprises a pressure relief valve, and the heat management unit comprises a heat storage, a heat exchanger, or a combination of both, and wherein the fluid cycle comprising the overload protection unit is configured to at least partially absorb heat generated during turning of the propulsion unit.

FIELD

The present invention relates to a steering system of an azimuthingpropulsion system. Further, the present invention relates to anazimuthing propulsion system. In particular, aspects of the inventionrelate to a steering system of an azimuthing propulsion systemcomprising a shock absorption system. Additionally, the inventionrelates to a method for absorbing heat generated during an over torquesituation of a steering system of an azimuthing propulsion system.Furthermore, the present invention relates to a method for operating asteering system of an azimuthing propulsion system. Yet further, thepresent invention relates to a computer readable memory.

BACKGROUND

Document WO 2000/15495 A1 describes common propeller propulsion systemsof vessels such as passenger ships, ferries, cargo vessels, lighters,oil tankers, ice-breakers, off-shore vessels, etc. and propeller unitsin which the equipment creating the propulsion power for the propellershaft and any gearing are positioned outside the hull of the vesselwithin a special chamber, pod, or propulsion unit supported for rotatingin relation to the hull. The propeller unit can be also used forsteering the vessel instead of separate rudder gear. Generally, theseunits are referred to as azimuthing propulsion systems or rudderpropeller devices, and, e.g., the applicant in the present applicationprovides azimuthing units of this kind under the trademark “AZIPOD”.Presently, azimuthing propulsion systems with a power of more than 20 MWare being designed.

An azimuthing propulsion system includes one or several propulsionpropellers mounted on a shaft journalled in the propulsion unit, whichis substantially turnable around a vertical axis. The propulsion unit isattached to the lower end of a shaft structure which is turnablyjournalled in the hull of the ship and is normally a straight tubularmember. By turning the so called turning shaft it is possible to directthe propulsion unit and thus also the propeller flow in any desireddirection.

The azimuthing propulsion system's steering arrangement has generallybeen implemented so that a geared tiller ring or the like tiller rim hasbeen attached to the tubular shaft which forms the system's swivellingaxis, which tiller is rotated with the aid of hydraulic or electricmotors adapted to cooperate with it.

In a case that a hydraulic turning system has been employed, theoperating machinery which creates the hydraulic pressure required in themotors comprises of one or more hydraulic pumps and of one or moreelectric motors. In order to enhance the service reliability of thesteering gear and for meeting the redundancy level required, thehydraulic motors can be arranged in two or more separate hydrauliccircuits, each of which can be separated from the system and put toidling in a case of malfunction.

In case of electric steering, the corresponding redundancy level andidling functions are gained by either direct connection of electricmotors to the tiller rim, or preferably via a reduction gear.

Normally, in operation the torque required for the turning of thepropulsion unit is dependent on the distance of the propeller plane fromthe so called turning axis or swivelling axis of the propulsion unit.Typically, the propeller is located at the end of the propulsion unit,and hence, is relatively far from the propulsion unit's turning axis.Consequently, a relatively high torque is required for turning thepropulsion unit. The steerability of a vessel equipped with anazimuthing propulsion system is excellent, but the torque required forturning the propulsion unit can be high and increases as a function ofthe propulsion power. The high torque causes problems in particular inslow moving ships with high propeller thrust such as tugs and icebreakers. The torque required for turning the propulsion unit can reachhigh values and thus requires a very strong steeling machinery. Further,over torque situations may, for example, occur due to collisions of atleast a part of the system with blocks of ice or other objects when thepropulsion unit is forced to turn along the colliding object in order toavoid damage.

A hydraulic turning system has been employed, because hydraulics readilyallow the relatively high torque required for turning an azimuthingpropulsion unit to be obtained at a relatively low speed of rotation. Atthe same time, the turning and steering of the vessel by means of thehydraulics can be readily and relatively precisely controlled with theaid of the traditional pumps and valve gears and corresponding hydrauliccomponents. Further, the shock-absorption- and torque limitationfeatures that are protecting the mechanical parts of the powertransmission of the steering system have most suitably been implementedwith hydraulics due to an excellent response time and accuracy of thehydraulic pressure relief valves. Hence, the hydraulic powertransmission system has been considered as the most suitable solutionfor steering systems that are frequently exposed for high external loadsthat are causing over torque situations.

The propulsion unit has to be able to turn along with a colliding objectso that no damage is caused to the steering system. The amount ofabsorbed heat corresponds to the loss energy that is created at thepressure relief valves when the propulsion unit is forced to turn by acolliding object. Traditionally, the azimuthing propulsion systems withhydraulic steering have four very large hydraulic motors directlyconnected to the steering gear including pinions. The pressure reliefvalves are preferably integrated to the same package with the motors, togain a standard solution with highly predictable dynamic properties. Thelarge motors are containing a sufficient oil volume to absorb the heatgenerated in an over torque situation. An over torque situation may, forexample, occur in arctic environments when the propulsion system isfrequently exposed to collisions with blocks of ice during operation.

Over-dimensioning of parts of the steering system should be avoided.However, use of smaller hydraulic motors operating at an increasedrotation speed compared to a system comprising the large hydraulicmotors can create a heating problem during an over torque movement dueto the small motor volume, the high rotation speed and small volumes inthe working lines between the pressure relief valves and the motorports.

In view of the foregoing, it would be beneficial to provide anazimuthing propulsion system or a steering system which comprises ashock absorption system that can absorb the heat generated during anover torque situation of a steering system of the propulsion system inorder to utilize small motors without running into heating problems.

SUMMARY OF THIS INVENTION

The invention is defined by the features of the independent claims. Somespecific embodiments are defined in the dependent claims.

According to a first aspect of the present invention, there is provideda steering system of an azimuthing propulsion system, the steeringsystem comprising at least one hydraulic motor configured to operate anazimuthing system of a propulsion unit the propulsion unit beingarranged outside a vessel, a fluid cycle from the at least one hydraulicmotor via a separate hydraulic overload protection unit and back to themotor, the overload protection unit comprises a pressure relief unit anda heat management unit and wherein the pressure relief unit comprises apressure relief valve and the heat management unit comprises a heatstorage, a heat exchanger, or a combination of both, and wherein thefluid cycle comprising the overload protection unit is configured to atleast partially absorb heat generated during turning of the propulsionunit.

Various embodiments of the first aspect may comprise at least onefeature from the following bulleted list:

-   -   the steering system is configured to allow the propulsion unit        to turn along with a colliding object, as a certain predefined        load pressure level has been exceeded    -   turning of the propulsion unit is caused by a critical torque        caused by an external force    -   the turning of the propulsion unit caused by an external force        represents an over torque situation of the steeling system    -   the heat storage comprises a pipeline or a temperature balance        tank or both    -   at least a part of the heat management unit is arranged in        series with the pressure relief unit    -   the temperature balance tank is configured to receive a heated        outlet fluid flow of the pressure relief valve and to provide a        filling fluid flow to a hydraulic volume where a hydraulic motor        inlet is connected to    -   a temperature of a temperature balance tank fluid outlet flow is        less than the temperature of a pressure relief fluid outlet flow    -   the temperature balance tank is configured to increase a        rotation volume of the fluid cycle    -   the temperature balance tank is configured to increase a heat        capacity of the fluid cycle    -   the steering system comprises hydraulic interconnections between        the at least one hydraulic motor and the overload protection        unit    -   the hydraulic interconnection, the at least one hydraulic motor        and the overload protection unit are configured to circulate a        fluid    -   the fluid cycle is configured to decrease a temperature of a        fluid in the fluid cycle by means of a heat sink or cooler    -   the temperature balance tank is configured to decrease a        temperature of a fluid in the fluid cycle by means of a heat        sink or cooler    -   the fluid cycle comprises a boost pressure fluid system coupled        to the overload protection unit    -   a gear is arranged between the at least one hydraulic motor and        a steering gear of the propulsion system    -   the heat management unit is separated from the pressure relief        unit    -   the heat management unit and the pressure relief unit are        integrated    -   the temperature of the temperature balance tank fluid outlet        flow is less than 10 [° C.], less than 15 [° C.], less than 20        [° C.], or less than 35 [° C.] than the temperature of the        pressure relief fluid outlet flow    -   a volume of the temperature balance tank is adapted to hold at        least 5 [l], at least 10 [l], at least 15 [l], or at least 20        [l] of fluid    -   the steering system is implemented in or coupled to an        azimuthing propulsion system

According to a second aspect of the present invention, there is providedan azimuthing propulsion system comprising at least one hydraulic motorconfigured to operate a azimuthing system of a propulsion unit, thepropulsion unit being arranged outside a vessel, a fluid cycle from theat least one hydraulic motor via a separate hydraulic overloadprotection unit and back to the motor, the overload protection unitcomprises a pressure relief unit and a heat management unit, and whereinthe pressure relief unit comprises a pressure relief valve, and the heatmanagement unit comprises a heat storage, a heat exchanger, or acombination of both, and wherein the fluid cycle comprising the overloadprotection unit is configured to at least partially absorb heatgenerated during turning of the propulsion unit.

Various embodiments of the second aspect may comprise at least onefeature from the following bulleted list:

-   -   the azimuthing, propulsion system is configured to allow the        propulsion unit to turn along with a colliding object, as a        certain predefined load pressure level has been exceeded    -   turning of the propulsion unit s caused by a critical torque        caused by an external force    -   the turning of the propulsion unit caused by an external force        represents an over torque situation of a steering system    -   at least a part of the heat management unit is arranged in        series with the pressure relief unit    -   the heat storage comprises a pipeline or a temperature balance        tank or both    -   the temperature balance tank is configured to receive a heated        outlet fluid flow of the pressure relief valve and to provide a        filling fluid flow to a hydraulic volume where a hydraulic motor        inlet is connected to    -   the temperature balance tank is configured to increase a        rotation volume of the fluid cycle    -   the temperature balance tank is configured to increase a heat        capacity of the fluid cycle    -   a temperature of a temperature balance tank fluid outlet flow is        less than the temperature of a pressure relief fluid outlet flow    -   the system comprises hydraulic interconnections between the at        least one hydraulic motor and the overload protection unit    -   the hydraulic interconnection, the at least one hydraulic motor        and the overload protection unit are configured to circulate a        fluid    -   the fluid cycle comprises a boost pressure fluid system coupled        to the overload protection unit    -   a gear is arranged between the at least one hydraulic motor and        a steering gear of the propulsion system

According to a third aspect of the present invention, there is provideda method for absorbing heat generated during an over torque situation ofa steering system of an azimuthing, propulsion system, the methodcomprising allowing a propulsion unit to turn along with a collidingobject, the propulsion unit being arranged outside a vessel, circulatingfluid from a hydraulic motor via a separate hydraulic overloadprotection unit and back to the motor, and wherein the overloadprotection unit comprises a pressure relief unit and a heat managementunit, and wherein the pressure relief unit comprises a pressure reliefvalve, and the heat management unit comprises a heat storage, a heatexchanger, or a combination of both, and absorbing at least a part ofthe generated heat by means of the overload protection unit.

Various embodiments of the third aspect may comprise at least onefeature from the following bulleted list:

-   -   the method further comprising receiving a heated outlet fluid        flow of the pressure relief valve, and providing a filling fluid        flow to a hydraulic motor inlet volume    -   the method yet further comprising transferring heat away from        the fluid present in the overload protection unit by means of a        heat sink which is coupled to the heat storage or integrated in        the heat storage

According to a fourth aspect of the present invention, there is provideda method for operating an azimuthing propulsion system, the methodcomprising allowing a propulsion unit to turn, the propulsion unit beingarranged outside a vessel, circulating fluid from a hydraulic motor viaa pressure relief valve to a temperature balance tank and back to themotor, and absorbing at least a part of heat generated during an overtorque situation of a steering system of the propulsion system due to acollision of at least a part of the system with ice or any other objectby means of the temperature balance tank.

According to a fifth aspect of the present invention, there is provideda computer readable memory having stored thereon a set of computerimplementable instructions capable of causing a computing device, inconnection with an azimuthing propulsion system or in connection with asteering system 30 of an azimuthing propulsion system, to couple a heatexchanger to a fluid cycle based on a fluid temperature measurement in apart of an overload protection unit, or to control a fluid flow of acoolant of the heat exchanger coupled to the fluid cycle, based on afluid temperature measurement in a part of the overload protection unit,or to directly exchange fluid present in the overload protection unit bymeans of an actively controllable valve connection from a fluid volumeof a heat storage to a tank line or corresponding lower pressure line.

Considerable advantages are obtained by means of certain embodiments ofthe present invention. Certain embodiments of the present inventionprovide an azimuthing propulsion system. Certain other embodiments ofthe present invention provide a method for absorbing heat generatedduring an over torque situation of a steering system of an azimuthingpropulsion system. Additionally, certain other embodiments of thepresent invention provide a method for operating an azimuthingpropulsion system.

According to certain embodiments of the present invention, heatgenerated during an over torque situation of a steering system of anazimuthing propulsion system can be absorbed. Therefore, significantlysmaller hydraulic motors can be used in the system. Certain embodimentsof the present invention enable the use of relatively small hydraulicmotors on arctic vessels or ice breakers, for instance.

The small hydraulic motors are more compact than the motors currentlyused, thus reducing weight, dimensions and costs of the propulsionsystem. The availability and diversity of smaller motors is further muchbetter than of large ones on the market. The propulsion unit can bebuilt by using standard components without making any further changes tothe system. Additionally, the system can be manufactured in industrialscale.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of particular embodiments of thepresent invention and their advantages, reference is now made to thefollowing descriptions, taken in conjunction with the accompanyingdrawings. In the drawings:

FIG. 1 illustrates a schematic view of an azimuthing propulsion systemin accordance with at least some embodiments of the present invention,

FIG. 2 illustrates a schematic view of an azimuthing propulsion systemcomprising a heat sink in accordance with at least some embodiments ofthe present invention,

FIG. 3 illustrates a schematic view of an azimuthing propulsion systemcomprising a gear in accordance with at least some embodiments of thepresent invention,

FIG. 4 illustrates a schematic view of an fluid cycle diagram inaccordance with at least some embodiments of the present invention,

FIG. 5 illustrates a schematic view of a fluid cycle diagram of asteering system of an azimuthing propulsion system in accordance with atleast some embodiments of the present invention during an over torquesituation of the steering system,

FIG. 6 illustrates a schematic view of a fluid cycle diagram of asteering system of an azimuthing, propulsion system in accordance withat least some embodiments of the present invention during an over torquesituation of the steering system, and

FIG. 7 illustrates a schematic view of a fluid cycle diagram of asteering system of an azimuthing propulsion system comprising anoverload protection unit in accordance with at least some embodiments ofthe present invention.

EMBODIMENTS

Certain embodiments of the present invention relate to an azimuthingpropulsion system comprising a shock absorption system. The shockabsorption system is designed to absorb heat generated during an overtorque situation of a steering system of the propulsion system. Such anover torque situation may, for example, take place when at least a partof the propulsion system is exposed to collisions with blocks of ice orany other objects. The system is capable of absorbing such shocks byallowing the propulsion unit to turn along with the colliding object ina suitable direction and absorbing the generated heat.

In FIG. 1 a schematic view of an azimuthing propulsion system 1 inaccordance with at least some embodiments of the present invention isillustrated. The propulsion system 1 includes equipment for creating thepropulsion power for the propeller shaft and gearing positioned outsidethe hull 12 of a vessel within a special propulsion unit 3 supported forrotating in relation to the hull 12.

The azimuthing propulsion system 1 comprises a plurality of hydraulicmotors 2 configured to operate the steering system of the propulsionunit 3 which is arranged outside the vessel. The term “operate” meansthat the propulsion unit 3 of the propulsion system 1 can be turnedrelative to the hull 12 around a vertical axis of rotation. Typically,the propulsion unit 3 can be turned unlimitedly in both directionsrelative to the hull 12. The propulsion system may, for example, includefour or six hydraulic motors coupled to the steering gear of thepropulsion system 1. In FIG. 1 only one hydraulic motor 2 is shown.

The system 1 further includes a shock absorption system comprising afluid cycle from the hydraulic motor 2 via a pressure relief valve 5 toa temperature balance tank 6 and back to the motor 2. Typically, oil isused as fluid in the fluid cycle. The temperature balance tank 6 isconfigured to at least partially absorb heat generated during an overtorque situation of the steering system of the propulsion system 1. Thetemperature balance tank 6 may be also called fluid warren ortemperature stabilization reservoir, for instance. The hydraulic motor2, the pressure relief valve 5 and the temperature balance tank 6 ofeach fluid cycle are arranged inside the vessel.

For example, in case that at least a part of the propulsion system 1 isexposed to collisions with blocks of ice or any other object 14 duringoperation, the propulsion unit 3 is able to turn along with thecolliding object 14 so that no damage is caused to the steering system.Therefore, the pressure in the hydraulic motor 2 increases. At a certainpressure level the pressure relief valve 5 is opened as the workpressure exceeds the set pressure of the pressure relief valve. Such aturning of the propulsion unit 3 caused by an external force representsan over torque situation of the steering system, where the fluid of thehydraulic system is heated. A hydraulic motor fluid outlet flow 13 flowsfrom the hydraulic motor 2 to the pressure relief valve 5. Subsequently,the pressure relief valve fluid outlet flow 7 flows in the direction ofa heat storage such as the temperature balance tank 6 and/or a heatexchanger via piping 9. The temperature balance tank 6 represents asubstitute for a long pipeline and can act as a buffer volume for thehot pressure relief fluid outlet flow 7. The temperature balance tank 6may, for example, comprise a piping labyrinth in order to provide asubstitute for a long pipeline. Additionally, in the temperature balancetank 6 the temperature of the fluid may be reduced, for instance. Inother words, the temperature balance tank 6 may be configured todecrease a temperature of the heated incoming pressure relief valvefluid outlet flow 7. The amount of absorbed heat corresponds to the lossenergy that is created when the fluid is forced to flow through thepressure relief valve by the motor 2 that is acting as a pump as thepropulsion unit is forced to turn by the colliding object 14. Next, thetemperature balance tank fluid outlet flow 8 can flow back to thehydraulic motor 2. The temperature of the temperature balance tank fluidoutlet flow 8 returning to the hydraulic motor 2 is less than thetemperature of the pressure relief valve fluid outlet flow 7.

The temperature balance tank 6 increases the rotation volume of thefluid cycle. According to certain embodiments, the volume of thetemperature balance tank 6 is adapted to hold fluid in the range between5 [l] and 20 [l], for example at least 10 [l] or at least 15 [l]. Thetemperature of the temperature balance tank fluid outlet flow 8 isrelatively cool as long as the total capacity of the temperature balancetank 6 has not been significantly exceeded by the pressure relief valvefluid outlet flow 7.

It is noted, that instead of including a temperature balance tank 6between the pressure relief valve 5 and the hydraulic motor 2, only astraight or bended piping may be arranged between the pressure reliefvalve 5 and the hydraulic motor 2 in order to form a fluid circle. Thepiping may have a suitable cross-sectional area and/or length in orderto provide a sufficient fluid volume in the fluid cycle.

The system 1 is able to avoid a heating problem in the work line betweenthe pressure relief valve 5 and the motor port during an over torquesituation of the steering system. The fluid present in the fluid cyclecan circulate multiple times through the same loop from the hydraulicmotor 2, via the pressure relief valve 5, and via the temperaturebalance tank 6.

In FIG. 2 a schematic view of an azimuthing propulsion system 1comprising a heat sink 10 in accordance with at least some embodimentsof the present invention is illustrated. For example, a heat storage mayinclude a heat sink 10 comprising a piping system for guiding a workingfluid through the piping system, i.e. a gas or liquid can flow throughthe piping system of the heat sink 10 in order to transfer heat awayfrom the fluid present in the heat storage, e.g. the temperature balancetank 6. Typically, a liquid such as oil, water, or water-glycol mixtureis used as a working fluid.

According to other embodiments, the heat sink 10 may comprise coolingfins or other objects protruding away from the temperature balance tank6 in order to increase the effective area of heat transfer. Such coolingfins or objects protruding away from the temperature balance tank 6 maybe arranged instead of or in addition to a heat sink 10 comprising apiping system for guiding a working fluid through the piping system. Thecooling fins or objects protruding away from the temperature balancetank 6 may be, for example, made of copper, aluminium or any othermaterial having a suitable thermal conductivity.

According to another embodiment, a boost pressure fluid can flow throughthe temperature balance tank 6 so that it flushes the temperaturebalance tank 6 constantly. Of course, also such an active cooling systemmay further comprise cooling fins or objects protruding away from thetemperature balance tank 6.

The time period allowed in between successive ice collisions orcollisions with other objects 14 without overheating of the hydraulicsystem can be very short due to cooling the fluid present in thetemperature balance tank 6. Therefore, arctic vessels and ice breakersincluding an azimuthing propulsion system 1 for propulsion of the vesselmay comprise such a system for (actively) cooling the fluid present inthe temperature balance tank 6, for instance.

The system 1 is able to avoid a heating problem in the work line betweenthe pressure relief valve 5 and the motor port during an over torquesituation of the steering system. The fluid present in the fluid cyclecan circulate multiple times through the same loop from the hydraulicmotor 2, via the pressure relief valve 5, and via the temperaturebalance tank 6.

In FIG. 3 a schematic view of an azimuthing propulsion system 1comprising a gear 11 in accordance with at least some embodiments of thepresent invention is illustrated. The hydraulic motor 2 is coupled tothe steering gear of the propulsion system via a gear 11, for example aplanetary gear. The propulsion system 1 further additionally includes aheat storage, e.g. a temperature balance tank comprising a heat sink 10.

By means of placing the gear 11 between the hydraulic motor 2 and thepinions of the steering gear of the system 1, torque capacity demandscan be met while simultaneously using a smaller hydraulic motor. Thesystem 1 is also able to avoid a heating problem in the work linebetween the pressure relief valve 5 and the motor port during an overtorque situation of the steering system. The fluid present in the fluidcycle can circulate multiple times through the same loop from thehydraulic motor 2, via the pressure relief valve 5, and via thetemperature balance tank 6.

In FIG. 4 a schematic view of a fluid cycle diagram in accordance withat least some embodiments of the present invention is illustrated. Afluid cycle 4 from the hydraulic motor to the pressure relief valve tothe heat storage and back to the hydraulic motor is shown. The heatstorage may be a temperature balance tank 6, for instance.

In FIG. 5 a schematic view of a fluid cycle diagram of a steering system30 of an azimuthing propulsion system 1 in accordance with at least someembodiments of the present invention during an over torque situation ofthe steering system 30 is illustrated. The steering system 30 includes apump module 16 and a motor module 15.

The pump module 16 comprises an electric motor 17 which rotates ahydraulic pump 18. The pump module 16 may further comprise a boosterpump 19, filling functions 20, and flushing functions 21.

The motor module 15 comprises a hydraulic motor 2, which is coupled topinions 29 via a gear 11. The motor module 15 further comprises a fluidcycle 4 from the hydraulic motor 2 via the second pressure relief valve23 to a heat storage, for example a temperature balance tank 6, via thefirst filling check valve 24 and back to the motor 2. The motor module15 further comprises a first pressure relief valve 22 and a secondfilling check valve 25. The first pressure relief valve 22 and thesecond filling check valve 25 are not a part of the fluid cycle 4 duringover torque situation with counter-clockwise movement of the pinion 29as shown in FIG. 5. Additionally, motor module 15 comprises a valveconnection 26 which may be a shut-off valve or proportional valve, forinstance.

The booster pump 19 may be connected to the temperature balance tank 6via a booster line inlet check valve 27. The temperature balance tank 6can be constantly flushed with the fluid by means of the booster pump19.

In FIG. 6 a schematic view of a fluid cycle diagram of a steering system30 of an azimuthing propulsion system 1 in accordance with at least someembodiments of the present invention during an over torque situation ofthe steering system 30 is illustrated. The steering system includes apump module 16 and a motor module 15.

The pump module 16 comprises an electric motor 17 and a hydraulic pump18, and it may also comprise a booster pump 19, filling functions 20,and flushing functions 21.

The motor module 15 comprises a hydraulic motor 2, which is coupled topinions 29 via a gear 11. The motor module 15 further comprises a fluidcycle 4 from the hydraulic motor 2 via the first pressure relief valve22 to a heat storage, for example a temperature balance tank 6, via thesecond filling check valve 25 and back to the motor 2. The motor module15 further comprises a second pressure relief valve 23 and a firstfilling check valve 24. The second pressure relief valve 23 and thefirst filling check valve 24 are not a part of the fluid cycle 4 duringclockwise movement of the pinion 29 as shown in FIG. 6.

The booster pump 19 may be connected to the temperature balance tank 6via a booster line inlet check valve 27. The temperature balance tank 6can be constantly flushed with the fluid by means of the booster pump19.

The steering system 30 further comprises a computing device 31. There isprovided a computer readable memory having stored thereon a set ofcomputer implementable instructions capable of causing a computingdevice 31, in connection with an azimuthing propulsion system 1 or inconnection with a steering system 30 of an azimuthing propulsion system1, to couple a heat exchanger to a fluid cycle 4 based on a fluidtemperature measurement in a part of an overload protection unit, or tocontrol a fluid flow of the coolant of the heat exchanger, coupled tothe fluid cycle, based on a fluid temperature measurement in a part ofthe overload protection unit, or to directly exchange fluid present inthe overload protection unit by means of an actively controllable valveconnection 26 from a fluid volume of the heat storage to a tank line orcorresponding lower pressure line. The valve connection 26 may be ashut-off valve or proportional valve, for instance.

In FIG. 7 a schematic view of a fluid cycle diagram of a steering system30 of an azimuthing propulsion system 1 comprising an overloadprotection unit 32 in accordance with at least some embodiments of thepresent invention. The steering system 30 comprises at least onehydraulic motor 2 configured to operate an azimuthing, system of apropulsion unit 3 which is arranged outside a vessel. The steeringsystem 30 further includes a fluid cycle 4 from the at least onehydraulic motor 2 via a separate hydraulic overload protection unit 32and back to the motor 2. The overload protection unit (32) is a part ofthe fluid cycle (4). In other words, the fluid cycle (4) comprises theoverload protection unit (32). The overload protection unit 32 comprisesa pressure relief unit 34 and a heat management unit 33. The pressurerelief unit 34 comprises a pressure relief valve 5, and the heatmanagement unit 33 comprises a heat storage, a heat exchanger, or acombination of both. The fluid cycle 4 is configured to at leastpartially absorb heat generated during turning of the propulsion unit 3.

The steering system 30 is configured to allow the propulsion unit 3 toturn along with a colliding object. The turning of the propulsion unit 3is caused by a critical external force. The turning of the propulsionunit 3 caused by the external force represents an over torque situationof the steering system 30. At least a part of the heat management unitis arranged in series with the pressure relief unit. The heat storagemay comprise a pipeline or a temperature balance tank 6 or both, forinstance. The temperature balance tank 6 is configured to receive aheated outlet fluid flow of the pressure relief valve 5 and to provide afilling fluid flow to a hydraulic volume where a hydraulic motor inletis connected to. A temperature of a temperature balance tank fluidoutlet flow 8 is less than the temperature of a pressure relief fluidoutlet flow 7. The steering system 30 comprises hydraulicinterconnections between the at least one hydraulic motor 2 and theoverload protection unit 32. The hydraulic interconnection, the at leastone hydraulic motor 2 and the overload protection unit 32 are configuredto circulate a fluid.

It is to be understood that the embodiments of the invention disclosedare not limited to the particular structures, process steps, ormaterials disclosed herein, but are extended to equivalents thereof aswould be recognized by those ordinarily skilled in the relevant arts. Itshould also be understood that terminology employed herein is used forthe purpose of describing particular embodiments only and is notintended to be limiting.

Reference throughout this specification to one embodiment or anembodiment means that a particular feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment. Where reference is made to a numerical value using a termsuch as, for example, about or substantially, the exact numerical valueis also disclosed.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary. In addition, various embodiments and example of the presentinvention may be referred to herein along with alternatives for thevarious components thereof. It is understood that such embodiments,examples, and alternatives are not to be construed as de factoequivalents of one another, but are to be considered as separate andautonomous representations of the present invention.

Furthermore, the described features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments. In thisdescription, numerous specific details are provided, such as examples oflengths, widths, shapes, etc., to provide a thorough understanding ofembodiments of the invention. One skilled in the relevant art willrecognize, however, that the invention can be practiced without one ormore of the specific details, or with other methods, components,materials, etc. In other instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringaspects of the invention.

While the forgoing examples are illustrative of the principles of thepresent invention in one or more particular applications, it will beapparent to those of ordinary skill in the art that numerousmodifications in form, usage and details of implementation can be madewithout the exercise of inventive faculty, and without departing fromthe principles and concepts of the invention. Accordingly, it is notintended that the invention be limited, except as by the claims setforth below.

The verbs “to comprise” and “to include” are used in this document asopen limitations that neither exclude nor require the existence of alsoun-recited features. The features recited in depending claims aremutually freely combinable unless otherwise explicitly stated.Furthermore, it is to be understood that the use of “a” or “an”, thatis, a singular form, throughout this document does not exclude aplurality.

INDUSTRIAL APPLICABILITY

At least some embodiments of the present invention find industrialapplication in propulsion of arctic vessels and ice breakers.

REFERENCE SIGNS LIST

-   1 propulsion system-   2 hydraulic motor-   3 propulsion unit-   4 fluid cycle-   5 pressure relief valve-   6 temperature balance tank-   7 pressure relief valve fluid outlet flow-   8 temperature balance tank fluid outlet flow-   9 piping-   10 heat sink-   11 gear-   12 hull-   13 hydraulic motor fluid outlet flow-   14 object-   15 motor module-   16 pump module-   17 electric motor-   18 hydraulic pump-   19 booster pump-   20 filling functions-   21 flushing functions-   22 first pressure relief valve-   23 second pressure relief valve-   24 first filling check valve-   25 second filling check valve-   26 valve connection-   27 booster line inlet check valve-   28 flushing flow metering orifice-   29 pinion-   30 steering system-   31 computing device-   32 overload protection unit-   33 heat management unit-   34 pressure relief unit

CITATION LIST

-   Patent Literature-   WO 2000/15495 A1-   Non Patent Literature

1. A steering system of an azimuthing propulsion system, the steeringsystem comprising: at least one hydraulic motor configured to operate anazimuthing system of a propulsion unit, the propulsion unit beingarranged outside a vessel, and a fluid cycle from the at least onehydraulic motor via a separate hydraulic overload protection unit andback to the motor, wherein the overload protection unit comprises apressure relief unit and a heat management unit, and wherein thepressure relief unit comprises a pressure relief valve, and the heatmanagement unit comprises a heat storage, a heat exchanger, or acombination of both, and wherein the fluid cycle comprising the overloadprotection unit is configured to at least partially absorb heatgenerated during turning of the propulsion unit.
 2. The steering systemaccording to claim 1, wherein the steering system is configured to allowthe propulsion unit to turn along with a colliding object.
 3. Thesteering system according to claim 1, wherein turning of the propulsionunit is caused by a critical torque caused by an external force.
 4. Thesteering system according to claim 1, wherein the turning of thepropulsion unit caused by an external force represents an over torquesituation of the steering system.
 5. The steering system according toclaim 1, wherein at least a part of the heat management unit is arrangedin series with the pressure relief unit.
 6. The steering systemaccording to claim 1, wherein the heat storage comprises a pipeline or atemperature balance tank or both.
 7. The steering system according toclaim 6, wherein the temperature balance tank is configured to receive aheated outlet fluid flow of the pressure relief valve and to provide afilling fluid flow to a hydraulic motor inlet volume.
 8. The steeringsystem according to claim 6, wherein the temperature balance tank isconfigured to increase a rotation volume of the fluid cycle.
 9. Thesteering system according to claim 6, wherein the temperature balancetank is configured to increase a heat capacity of the fluid cycle. 10.The steering system according to claim 1, wherein a temperature of atemperature balance tank fluid outlet flow is less than the temperatureof a pressure relief fluid outlet flow.
 11. The steering systemaccording to claim 1, wherein the steering system further compriseshydraulic interconnections between the at least one hydraulic motor andthe overload protection unit.
 12. The steering system according to claim11, wherein the hydraulic interconnection, the at least one hydraulicmotor and the overload protection unit are configured to circulate afluid.
 13. The steering system according to claim 1, wherein the fluidcycle comprises a boost pressure fluid system coupled to the overloadprotection unit.
 14. The steering system according to claim 1, wherein agear is arranged between the at least one hydraulic motor and a steeringgear of the propulsion system.
 15. The steering system according toclaim 1, wherein the heat management unit is separated from the pressurerelief unit or the heat management unit and the pressure relief unit areintegrated.
 16. An azimuthing propulsion system comprising: at least onehydraulic motor configured to operate a azimuthing system of apropulsion unit, the propulsion unit being arranged outside a vessel,and a fluid cycle from the at least one hydraulic motor via a separatehydraulic overload protection unit and back to the motor, wherein theoverload protection unit comprises a pressure relief unit and a heatmanagement unit, and wherein the pressure relief unit comprises apressure relief valve, and the heat management unit comprises a heatstorage, a heat exchanger, or a combination of both, and wherein thefluid cycle comprising the overload protection unit is configured to atleast partially absorb heat generated during turning of the propulsionunit.
 17. The azimuthing propulsion system according to claim 16,wherein the azimuthing propulsion system is configured to allow thepropulsion unit to turn along with a colliding object.
 18. Theazimuthing propulsion system according to claim 16, wherein turning ofthe propulsion unit is caused by a critical torque caused by an externalforce.
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 30. A method for absorbing heat generatedduring an over torque situation of a steering system of an azimuthingpropulsion system, the method comprising: allowing a propulsion unit toturn along with a colliding object, the propulsion unit being arrangedoutside a vessel, circulating fluid from a hydraulic motor via aseparate hydraulic overload protection unit and back to the motor, andwherein the overload protection unit comprises a pressure relief unitand a heat management unit, and wherein the pressure relief unitcomprises a pressure relief valve, and the heat management unitcomprises a heat storage, a heat exchanger, or a combination of both,and absorbing at least a part of the generated heat by means of theoverload protection unit.
 31. The method according to claim 30, furthercomprising: receiving a heated outlet fluid flow of the pressure reliefvalve, and providing a filling fluid flow to a hydraulic volume where ahydraulic motor inlet is connected to.
 32. (canceled)
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